1. Field of the Invention
The invention relates generally to the field of photography, and particularly to a film cassette containing roll film.
2. Description of the Prior Art
In conventional 35 mm film manufacturers' cassettes, such as manufactured by Eastman Kodak Co. and Fuji Photo Film Co. Ltd., the filmstrip is wound on a flanged spool which is rotatably supported within a cylindrical shell. A leading section of the filmstrip approximately 21/3 inches long, commonly referred to as a "film leader", protrudes from a light-trapped slit or mouth of the cassette shell. One end of the spool has a short axial extension which projects from the shell, enabling the spool to be turned by hand. If the spool is initially rotated in an unwinding direction, the film roll inside the sheel will tend to expand radially since the inner end of the filmstrip is attached to the spool, and the fogged leader section protruding from the slit will remain stationary. The film roll can expand radially until a firm non-slipping relation is established between its outermost convolution and the inner curved wall of the shell. Once this non-slipping relation exists, there is a binding effect between the film roll and the shell which prevents further rotation of the spool in the unwinding direction. Thus, rotation of the spool in the unwinding direction cannot serve to advance the filmstrip out of the shell, and it is necessary in the typical 35 mm camera to engage the protruding leader section to draw the filmstrip out of the shell.
A 35 mm film cassette has been proposed which, unlike conventional film cassettes, can be operated to automatically advance the filmstrip out of the cassette shell by rotating the film spool in the unwinding direction. The film leader normally does not protrude from the cassette shell. Specifically, in U.S. Pat. No. 4,423,943, granted Jan. 3, 1984, there is disclosed a film cassette wherein the outermost convolution of the film roll wound on the film spool is radially constrained by respective circumferential lips of two axially spaced flanges of the spool to prevent the outermost convolution from contacting an inner curved wall of the cassette shell. The trailing end of the filmstrip is secured to the film spool, and the leading end of the filmstrip is slightly tapered along one longitudinal edge purportedly to allow it to freely extend from between the circumferential lips and rest against the shell wall. During initial unwinding rotation of the film spool, the leading end of the filmstrip is advanced along the shell wall until it reaches an entry to a film passageway in the cassette shell. Then, it is advanced into and through the film passageway to the outside of the cassette shell. The passageway has a width that is slightly less than the width of the filmstrip, thus resulting in the filmstrip being transversely bowed as it is uncoiled from the film spool, and thereby facilitating movement of the film edges under the circumferential lips of the respective flanges. However, severe transverse bowing of the filmstrip to move its longitudinal edges under the circumferential lips results in increased friction between the filmstrip and the cassette shell which will impede advance of the filmstrip from the shell and may damage the filmstrip. Another problem arises from the fact that the leading end of the filmstrip appears to be approximately 11/8 inch to 11/4 inch long (judging by the number of perforations illustrated in the leading end). In a high temperature and/or humidity environment, the ability of the filmstrip to clock-spring outwardly when coiled about the spool is lessened, and there is more of a tendency of the filmstrip to curl inwardly. Consequently, the leading end because of its length may curl away from the shell wall, whereupon it may be unable to gain access to the film passageway when the film spool is rotated in the unwinding direction.
3. The Cross-Referenced Applications
Like the type of film cassette disclosed in U.S. Pat. No. 4,423,923, cross-referenced application Ser. Nos. 173,396 and 193,323, each disclose a film cassette that contains a non-protruding film leader which is automatically advanced to the outside of the cassette shell in response to rotation of the film spool in the unwinding direction. Specifically, there is disclosed a film cassette wherein a convoluted film roll is wound on a spool between a pair of coaxially spaced, independently rotatable flanges. The two flanges have respective circumferential annular lips which prevent the outermost convolution of the film roll, including its leading end, from clock-springing into contact with the interior wall of the cassette shell. When the spool is initially rotated in the unwinding direction, the flanges momentarily remain stationary and the film roll, since its inner end is secured to the spool, tends to expand radially to ensure a firm non-slipping relation between the outermost convolution and the annular lips. Once the non-slipping relation exists, continued rotation of the spool will similarly rotate the flanges. This allows stationary internal spreaders to deflect successive portions of the annular lips to an axial dimension exceeding the film width, in turn allowing the leading end to be freed from the radial confinement of the annular lips and to be advanced through a film passageway to the outside of the cassette shell. A stripper-guide is located immediately in front of the film passageway to divert the leading end from the flanges and into the film passageway. To accomplish this, the stripper-guide must be received between the leading end and the next-inward convolution of the film roll. If, however, the film cassette is used during extremely high temperature and/or humidity conditions, it might be possible that the leading end after being freed from the radial confinement of the annular lips will retain a great deal of inward curl. Consequently, the leading end will not have sufficient clock-spring to separate sufficiently from the next-inward convolution to move within range of the stripper-guide. Thus, when the spool is rotated in the unwinding direction, the leading end may not be picked up by the stripper-guide and guided into the film passageway.
The film cassette disclosed in cross-referenced application Ser. No. 07/296,551 like the ones disclosed in the first two cross-referenced applications contains a non-protruding film leader which is automatically advanced to the outside of the cassette shell in response to unwinding rotation of the spool. In this example, a leading section of the film roll is tapered assymetrically and is relatively short to allow it to normally protrude from between the annular lips of the flanges and into slight contact with the interior wall of the cassette shell. When the spool is initially rotated in the unwinding direction, it is rotated relative to the flanges until there results a firm non-slipping relation between the outermost convolution and the flanges, to thus cause the flanges to be rotated thereafter with the spool. The non-slipping relation imparts a beam strength or longitudinal rigidity to the leading section of the film roll in contact with the shell wall which, when the spool is rotated in the unwinding direction, combines with the assymetric design of the leading section to increase the likelihood that the leading section will feed into the film passageway. If, however, the leading section has been stored between the stripper-guide and the next-inward convolution of the film roll, particularly for an extended time, rather than being stored in contact with the interior wall of the cassette shell, it could develop sufficient inward curl to keep it out of range of the stripper-guide. Consequently, when the spool is rotated in the unwinding direction, the leading section will not be picked up by the stripper-guide and guided into the film passageway.